The present invention relates to a coating method for simultaneously forming a plurality of organic-solvent-based magnetic layers on a continuously moving nonmagnetic support.
The recent trend has been to make magnetic recording media higher in density and smaller in thickness. To accomplish this, the art has gone from a single-layer to a multi-layer arrangement wherein a plurality of magnetic layers are formed on a nonmagnetic support.
Compared with a magnetic recording medium having a single magnetic layer, a magnetic recording medium having a plurality of magnetic layers has much better magnetic recording characteristics, for instance, a better magnetic data storage capacity. Thus, there has been a demand for the provision of magnetic recording media having two or more magnetic layers.
In general, formation of a plurality of magnetic layers on a support is carried out by, as is disclosed, for instance, by Japanese Patent Application Publications Nos. 43362/1979 and 43816/1983 and Japanese Published Unexamined Patent Applications Nos. 119204/1976, 51908/1977 and 16604/1978, wherein coating solutions such as those described above are applied onto a support and the solution layers are dried one by one to form a multi-layer structure.
However, the conventional method is disadvantageous in that since coating solution applying and drying operations must be repeatedly carried out, the productivity of the overall process is undesirably low, the coating apparatus is unavoidably bulky, and accordingly the cost of the equipment is high. In addition, sometimes irregularities occur in the interfaces between the various layers formed on the support, with the result that unwanted tape modulation noise is liable to occur. There has thus been a strong demand for the provision of a method for forming multiple layers on a support using only a single coating solution applying and drying step. In order to meet this requirement, Japanese Published Unexamined Patent Applications Nos. 212933/1987 and 124631/1987 have disclosed a method for simultaneously forming a plurality of magnetic layers on a support by coating. However, this method is still disadvantageous in that, in the case when nonmagnetic coating solutions and organic-solvent-based or other magnetic solutions are simultaneously applied to the support to form multiple layers thereon, depending on the compositions of the coating solutions, the coating solution for the upper layer may not be uniformly applied to the support, as a result of which the desired two layers cannot be formed properly, or it is completely impossible to apply the coating solution for the upper layer to the lower layer on the support. This difficulty is significant when the quantity of coating solution per unit area (i.e., the coating rate) for the upper layer is low, for instance, 15 cc/m.sup.2 or less, more particularly 10 cc/m.sup.2 or less, or when the coating speed is high, for example, about 100 m/min or more. On the other hand, if the layers have a thickness such that they can more easily be formed on the support, the surface of the resultant magnetic tape may become unacceptable. That is, if such a tape is used as a magnetic video tape, the reproduced signals will suffer from high noise level.
Another example of a multi-layer coating method has been proposed, for instance, in Japanese Unexamined Published Patent Application No. 126648/1985 for manufacture of photographing photosensitive materials. In accordance with this method, the coating solution for the upper layer is made to have a lower surface tension than that of the lower layer.
However, it is difficult to accurately measure the surface tension of each of the coating solutions forming the magnetic recording medium. Moreover, the above-described coating method is not suitable as a coating method for manufacturing magnetic recording media since the above-described magnetic coating solutions have a considerably high viscosity and are liable to condense and dry because they are organic solvent dispersion liquids. Therefore, measurement of the surface tension causes the solution surface to become wavy, as a result of which the ring or glass plate of the surface tension meter cannot be uniformly brought into contact with the solution surface; that is, it is difficult to accurately measure the surface tension. Furthermore, a magnetic coating solution obtained immediately after agitation at high speed and the same magnetic coating solution allowed to stand have significantly different viscosities, and accordingly different surface tensions, which adversely affects the measurement.
In order to decrease the surface tension of the upper layer described above, the aforementioned Japanese Published Unexamined Patent Application No. 126648/1985 has disclosed a method of using surface active agents, namely, those of the anion, cation and non-ion types, and amphoteric surface active agents. However, the surface active agents exhibit no great surface activity for organic-solvent-based magnetic coating solutions since the two differ in polar and oily groups, and thus are generally unable to greatly vary the surface tension of the magnetic coating solution.